The parliamentary intelligence and security committee (ISC) has postponed questioning the intelligence agencies in front of the television cameras this week, delaying the unprecedented event until after the summer recess, because it says it is too busy focusing on the murder of Lee Rigby and revelations about GCHQ's activities.

Nick Clegg, the deputy prime minister, appeared to have been unaware of the postponement as he revealed at his monthly press conference that the agencies were to give evidence to the cross-party committee on Thursday. He cited the event as a sign of the agencies' commitment to public accountability.

The committee said the session had been postponed because it wanted to focus on the recent revelations about GCHQ's activities, and the intelligence agencies' failure to track the alleged murderers of Lee Rigby, who was killed outside Woolwich barracks. It did not say why it could not have pursued these issues in public with the agencies on Thursday, arguably an ideal time to question them.

Clegg said: "I cannot stress enough how important I think it that the ISC has got the new powers. Don't underestimate what a break with convention it is that you have got the heads of the intelligence agencies going to the ISC this Thursday to give televised evidence."

He added: "All of us in senior positions across government have urged the intelligence agencies to account for themselves because that is what accountability is all about to the ISC."

Asked if he was surprised by the capacity of GCHQ to track telecommunications, he said: "I don't think we should be surprised at all that our intelligence agencies use technology to keep us safe. The question is: do they do that in a way that is proportionate, accountable and doesn't unduly trample on people's liberty and privacy?"

The committee has said nothing on the string of revelations flowing from documents released by Edward Snowden since it issued a one-line sentence a month ago saying it was waiting for a report from the intelligence agencies on GCHQ.

The absence of a political debate in the UK on the issues contrasts starkly with what has happened in the US and Germany. The US president, Barack Obama, has actively welcomed the debate and defended the role of the agencies.

It is the first time a British minister has admitted there is political and diplomatic concern at the reports on the scale of UK intelligence operations, including in Germany and the European Union.

Clegg said he was not going to comment on leaks or give a running commentary on intelligence matters. But he claimed: "I can tell you that my experience of being in government is that the checks and balances that exist to make sure that the intelligence that is gathered by British intelligence are second to none."

He claimed those constraints had been dramatically strengthened by the coalition government when it transformed the powers of the ISC, including giving it unprecedented powers to call for evidence from the intelligence agencies. He said that "for the first time the ISC can initiate investigations without asking permission of the government or the agencies themselves".

"I have an old-fashioned liberal belief that, yes, of course it is necessary for our security that things are done in secret, but we have got to make sure we have got belt and braces to make sure things are done lawfully."

He said he had personally verified that the checks and balances existed, including direct ministerial oversight in the field of communications intelligence.

"I still believe the powers on the statute book are considerable, proportionate and allow the agencies to do their work," he added.